This invention relates to Coriolis oscillating gyroscopes (gyros) and Coriolis closed loop accelerometers based on tuning fork configurations. This invention also relates to gyros and accelerometers which are planar and can be fabricated using MEMS (Microelectromechanical Systems) technologies. This invention also relates to planar designs which can be combined to form full inertial measurement units in the plane.
Coriolis double-ended tuning fork gyros are designed in planar form to take advantage of MEMS fabrication approaches. (See, for example, U.S. Pat. No. 5,349,855, the disclosure of which is incorporated herein by reference). They require oscillatory excitation of the tines which is done relative to the substrate. In response to rotation rate of the vehicle, the structure to which the tines are attached oscillates with an amplitude of oscillation proportional to the input rate. Presently the output motion is measured relative to the substrate. With both the driving and sensing functions being done relative to the substrate (or case), there is potential for mechanical and electrical coupling between the two members which can affect the performance.
Double-ended tuning fork gyros with the planar form do not have the capability to sense rotation about the axis normal to the plane. This planar configuration is necessary to allow a set of three planar gyros to sense rotation about the three orthogonal axes. A planar set of gyros allows the fabrication of a planar inertial measurement unit (IMU) when combined with a planar set of accelerometers.
Closed-loop, pendulous accelerometers based on the rebalance torque from tuning fork gyros have not been devised. The POGA (Pendulous Oscillating Gyro Accelerometer) concept disclosed in U.S. Pat. No. 5,457,993 (also incorporated by reference) needs to be extended for implementation with tuning fork gyros. In planar form these designs are complementary to the gyros above and can provide the planar accelerometers for the planar IMU.
It is therefore an object of this invention to devise tuning fork gyros with output gimbals to de-couple the drive of the tines from the sensing of the output motion by driving the tines relative to the gimbal and sensing the gimbal output motion relative to the case.
It is a further object of this invention to devise a planar tuning fork gyro with rotation rate sensitivity about the normal to the plane.
It is a further object of this invention to devise a closed loop, pendulous accelerometer with rebalance torque provided from a Coriolis gyro oscillated about its input axis.
It is a further object of this invention to devise planar, closed loop, pendulous accelerometers with rebalance provided by a Coriolis gyro oscillated about its input axis.
It is a further object of this invention to devise planar, closed loop, pendulous accelerometers with sensitivity about the three orthogonal axes.
It is a further object of this invention to devise planar gyros and accelerometers which can be combined in the plane to sense all six degrees of freedom.
The Coriolis Oscillating Gyro (COG) utilizes a double ended tuning fork (DETF) rotor that is attached rigidly to a gimbal called a torque summing member (TSM). The TSM is flexurally mounted to the case. The tines of the DETF are driven relative to the TSM. The TSM output motion is oscillatory and its motion is measured relative to the case. In this arrangement, the tine drive and output sensing are mechanically and electrically decoupled. The contribution to existing, DETF-based gyros is the addition of the gimbal.
The Coriolis Pendulous Oscillating Gyro Accelerometer (CPOGA) combines a pendulous accelerometer with a gyroscope. The pendulum rotates about its pendulous axis under the influence of acceleration input, and the gyro provides the rebalance torque to hold the pendulum at null; the accelerometer is operated closed loop. The principle is the same as used with the POGA.
The CPOGA is formed by adding a mass to the COG gimbal so as to make the gimbal pendulous. The CPOGA also requires that in addition to oscillation of the DETF tines, the COG is oscillated about its input axis by oscillating the gimbal. The interaction between the tines and gimbal oscillations generates the gyro torque. The gyro torque and pendulous torque both act on the gimbal and that is why the gimbal is referred to as the torque summing member.
For the gyro torque to be provided in the CPOGA, the preferred implementation is to maintain the tine oscillatory amplitude and TSM oscillatory amplitude constant. The frequency of oscillation for both needs to be the same. The gyro torque is generated by varying the phase between the oscillation of the tines and the oscillation of the gimbal. When the phase between them is 90xc2x0 the gyro torque is zero.
The gimbal allows for the addition of pendulous mass without affecting the DETF dynamics.
One difference between the gyros in the POGA and those in the CPOGA is that the POGA gyros are based on a single rotor mass oscillating in angle whereas for the CPOGA, two rotor masses oscillate along a straight line in a double-ended tuning fork arrangement.
A second difference between the CPOGA and POGA is that the POGA is based on three axes and three members which rotate about them, whereas the CPOGA is based on two axes and two elements (the tine rotor and gimbal) and the tines move along the first axis and the gimbal rotates about the second.
Planar designs of both the COG and CPOGA are possible. They enable fabrication by MEMS technologies. Measurement of rotation rate about all three axes can be achieved with the two planar COG designs. Measurement of acceleration along all three axes can be achieved with two planar CPOGA designs. A full IMU can be carried out with separately assembled planar COGs and CPOGAs lying on the same plane. A full IMU can be fabricated simultaneously on the same wafer (or planar substrate).
This invention features a Coriolis Oscillating Gyroscopic (COG) instrument, comprising: a Double Ended Tuning Fork (DETF) having two stems and two tines; a Torque Summing Member (TSM) rigidly coupled to the DETF stems; means located at least partially on the TSM for vibrating the tines sinusoidally in opposition along a first axis, the tines"" motion having a constant amplitude, a frequency and a phase; a case; a plurality of flexures connecting the TSM to the case, to allow the TSM and the DETF to rotationally oscillate together relative to the case about a second axis transverse to the first axis; and means for resolving rotation of the TSM relative to the case.
The flexures may be co-linear along the second axis that serves as the gyroscopic input and output axes. The TSM may be mass imbalanced about the second axis, so that the TSM experiences a pendulous torque when accelerated along a third axis transverse to both the first and second axes, causing the TSM and the DETF to rotate together about the second axis. The instrument may further include means for rotationally sinusoidally oscillating the TSM and the DETF together about the second axis, the oscillation having a constant amplitude, a phase, and a frequency the same as the tines"" vibrational frequency. The instrument may still further include means for resolving the phase difference between the tines"" vibration along the first axis and the TSM rotational oscillation about the second axis. The instrument may also include means for altering the phase relationship between the tines"" motion and the TSM and DETF oscillation, to generate a gyroscopic torque that balances the pendulous torque. The instrument may still further include means, responsive to the means for resolving the phase difference, for determining the instrument acceleration along the third axis. The first and second axes may be orthogonal.
In this embodiment, the instrument may further include means, responsive to the means for resolving rotation, for determining the TSM oscillation amplitude. The instrument may still further include means, responsive to the means for determining the TSM oscillation amplitude, for determining the rate of rotation of the instrument about the input axis.
The flexures may be spaced circumferentially about the TSM in a common plane, and allow the TSM and the DETF to together rotationally oscillate in the plane about the second axis that serves as the gyroscopic input and output axes. In this embodiment, the instrument may have a third, acceleration input axis, and the TSM is mass imbalanced about the second axis, so that the TSM experiences a pendulous torque when accelerated along the third axis, causing the TSM and the DETF to rotate together about the second axis. This instrument may further include means, responsive to the means for resolving, for rotationally sinusoidally oscillating the TSM and the DETF together about the second axis, the oscillation having a constant amplitude, a phase, and a frequency the same as the tines"" vibrational frequency. This instrument may still further include means for resolving the phase difference between the tines"" vibration along the first axis and the TSM rotational oscillation about the second axis. This instrument may also include means for altering the phase relationship between the tines"" motion and the TSM and DETF oscillation, to generate a gyroscopic torque that balances the pendulous torque. This instrument may further include means, responsive to the means for resolving the phase difference, for determining the instrument acceleration along the third axis. The first and second axes may be orthogonal.
This embodiment may further include means, responsive to the means for resolving rotation, for determining the TSM oscillation amplitude. The embodiment may further include means, responsive to the means for determining the TSM oscillation amplitude, for determining the rate of rotation of the instrument about the input axis. The first and second axes may be mutually orthogonal.
The means for resolving rotation of the TSM relative to the case may be at least partially located on the case. The means for rotationally sinusoidally oscillating the TSM and the DETF together may be at least partially located on the case. The means for rotationally sinusoidally oscillating the TSM and the DETF together may be at least partially located on the case. The tines may comprise parallel plates. The DETF, the TSM and the flexures may be located in the same plane to define an essentially planar instrument. The TSM may be essentially cylindrical to define a three dimensional instrument.
In another embodiment, this invention features a Coriolis Oscillating Gyroscopic instrument, comprising: a Double Ended Tuning Fork (DETF) having two stems and two tines; means for vibrating the tines sinusoidally in opposition along a first axis, the tines"" motion having a constant amplitude, a frequency and a phase; a case; flexures connecting the DETF to the case and spaced circumferentially about the DETF in a common plane, to allow the DETF to rotationally oscillate in the plane about a second axis that serves as the gyroscopic input and output axis, and that is orthogonal to the first axis; and means for resolving rotation of the DETF about the second axis.
In this embodiment, the instrument may further include means, responsive to the means for resolving rotation, for determining the DETF oscillation amplitude. This instrument may still further include means, responsive to the means for determining the DETF oscillation amplitude, for determining the rate of rotation of the instrument about the input axis.
In another embodiment, this invention features an essentially planar inertial measurement unit for measuring motion in six degrees of freedom, comprising: three planar gyroscopes, each for measuring rotations about one of three mutually orthogonal axes; and three planar accelerometers, each for measuring acceleration along one of the three mutually orthogonal axes.
In yet another embodiment, this invention features a Coriolis Oscillating Gyroscopic instrument, comprising: a Double Ended Tuning Fork (DETF) having two stems and two tines; means for vibrating the tines sinusoidally in opposition along a first axis, the tines"" motion having a constant amplitude, a frequency and a phase; a case; a plurality of flexures connecting the DETF to the case, to allow the DETF to rotationally oscillate relative to the case about a second axis transverse to the first axis; wherein the DETF is mass imbalanced about the second axis, so that the DETF experiences a pendulous torque when accelerated along a third axis, causing the DETF to rotate about the second axis; means for rotationally sinusoidally oscillating the DETF about the second axis, the oscillation having a constant amplitude, a phase, and a frequency the same as the tines"" vibrational frequency; means for resolving rotation of the DETF relative to the case; means for resolving the phase difference between the tines"" vibration along the first axis and the DETF rotational oscillation about the second axis; means for altering the phase relationship between the tines"" motion and the DETF oscillation, to generate a gyroscopic torque that balances the pendulous torque; and means, responsive to the means for resolving the phase difference, for determining the instrument acceleration along the third axis.
Another embodiment features a stacked Coriolis Oscillating Gyroscopic instrument, comprising: a plurality of stacked, planar Coriolis Oscillating Gyroscopic instruments, each one comprising: a Double Ended Tuning Fork (DETF) having two stems and two tines; a Torque Summing Member (TSM) rigidly coupled to the DETF stems; a case; and a plurality of flexures connecting the TSM to the case, wherein the flexures are spaced circumferentially about the TSM in a common plane, and allow the TSM and the DETF to together rotationally oscillate in the plane about the second axis that serves as the gyroscopic input and output axes; wherein the cases are mechanically coupled, and the TSMs are mechanically coupled, to create a unitary stacked design; means located at least partially on each TSM for vibrating all of the tines sinusoidally in opposition, each vibrating along a first axis, the tines"" motions all having the same frequency and phase; and means for resolving rotation of each TSM relative to its respective case. This instrument may further include means for rotationally sinusoidally oscillating each TSM and DETF together about the second axis.